Turns out it's the length of the workout that matters.

I realized this anew last week, when I took advantage of the one glorious day of 60-degree weather to go outside and do some bag work. I had intended to go hard for 20 minutes, then come in and get some writing done. But as I neared the 30-minute mark I felt a familiar rush of energy and intensity that I’d been missing. That was when the real work started, when I really got into the rhythm of kicking and punching and pushed the cardio into high gear. I ended up working out for a little over an hour and feeling the best I’d felt in weeks.

According to Business Insider, it’s no surprise that an hour of kicking and punching yielded significantly better mood- and brain-boosting benefits. Although as little as 30 minutes of cardio activity have been proven to strengthen cognitive connectivity and protect against depression, the best results for the brain come from at least 45 minutes of combined cardio and resistance training.

Researchers still aren’t sure why this type of exercise appears to provide a boost to the brain, but some studies suggest it has to do with increased blood flow, which provides our minds with fresh energy and oxygen. One recent study in older women who displayed potential symptoms of dementia also found that aerobic exercise was linked with an increase in the size of the hippocampus, a brain area involved in learning and memory. Another reason might have to do with cardio’s ability to help reduce levels of the body’s natural stress hormones, such as adrenaline and cortisol, according to a recent study in the Journal of Physical Therapy Science.

It’s not exactly a surprise that more exercise yields better results, but for me at least, there’s a psychological component involved. No matter how much I psych myself up to go all out for 7 minutes, there’s a part of my brain that resists the effort. It’s like the way your muscles instinctively recoil against stretching when you’re not warmed up — it’s impossible to think your way out of. For me, it takes time to get over that resistance.

I’m usually about 20 to 30 minutes into a workout before I really start to throw myself into it, and it’s those last 30 minutes when I get the most work in. I tend to stop less for sips of water or to catch my breath, and as a result I end up sweating harder and wearing myself out. The amount of effort expended pays off in a better mood lift and a greater sense of accomplishment afterward — and, apparently, greater cognitive benefits.

So while the 7-minute workout works in a pinch, I’ll always be a devotee of the hour-long grind for better results, more sweat, more fun — and that extra boost of brain power.